1. Field of the Invention
This invention relates to a sound reproducing apparatus for use in a closed space, such as, for example, the passenger compartment of a vehicle.
2. Description of the Prior Art
Sound reproducing apparatuses such as compact disk players have been lately improved in performance, and there is an increasing demand for expansion of the sound range to reproduce super-bass sound with respect to low-range sound reproduction in the passenger compartment of a vehicle. With this background, a sound reproducing apparatus for use in the passenger compartment of a vehicle such as that shown in FIG. 14 has been known. In FIG. 14, reference numeral 16 denotes a speaker unit and reference numeral 17 denotes a rear trunk. In this example, the rear trunk 17 is used as an enclosed cavity on the rear side of the speaker unit 16 so as to expand the low frequency range of sound pressure frequency response in the passenger compartment of the vehicle.
FIG. 15 shows another example of a conventional apparatus which has a speaker system 18 of closed type having a small capacity or a system of bass-reflex type and an electric compensation means 19 such as a tone control or graphic equalizer.
In this example, the falling characteristic of the speaker system 18 in the low range is adjusted or improved by the electric compensation means 19 so as to expand the low frequency range of sound pressure frequency response in the passenger compartment. FIG. 16 shows an example of the frequency characteristic (a) of the speaker system and an example (b) of the electric compensation means.
However, the conventional arrangement shown in FIG. 14 cannot be applied to a type of vehicle such as a so-called hatch-back car or station wagon which is constructed without any closed rear trunk. Even when it is applied to the type of vehicle that has a rear trunk, the only feasible method of designing the interior sound pressure frequency characteristics is one wherein the constants of the speaker unit itself are changed. The degree of design freedom for this arrangement is thus very restricted.
The conventional arrangement shown in FIG. 15 is free from this problem, but it requires large input power for the speaker unit in the low frequency range because of the use of electric compensation means 19, thereby necessitating the provision of a relatively high power amplifier of. There is also a problem of distortion due to increased signal amplitude affecting the tone quality.